Mineralogy and Petrology of Ultramafic Xenoliths from Kharchinsky Volcano, Kamchatka

Abundant ultramafic xenoliths from Kharchinsky Volcano provide a rare opportunity to study the physical conditions and processes in the upper mantle and lower crust of the Kamchatka arc. Forty-five peridotite and pyroxenite xenoliths, which are representative of the 250 samples that were collected from an alkaline dike at the summit of the late Pleistocene volcano, have been studied petrographically and analyzed for their whole-rock major element and mineral compositions. Peridotite xenoliths are primarily harzburgites with protogranular textures and olivine and pyroxene compositions that are uniformly Mg-rich (Fo91-92, CPX Mg-no. 0.94-0.96). Spinel compositions are Cr-rich, and several samples contain mm-scale and veins of pargasitic amphibole. Kink banding in olivine is common in the peridotites, consistent with deformation under mantle conditions. Silicate mineral compositions in the pyroxenites are more Fe-rich and more variable (FO76-o76-82, CPX Mg-no. 0.75-0.88) than in the peridotites. Oxides compositions are also relatively Fe-rich (magnetite, hercynitic spinel). Pyroxenites contain up to 50% hornblende, which occurs in mm-scale veins and as coarse poikilitic crystals. Texturally, the pyroxenites, which show mm-scale layering in some large samples, are generally more finely crystalline and less intensely deformed than the peridotites. Two-pyroxene thermometry (Brey & Köhler, 1990, J. Petrology) indicates that the peridotites, which we interpret as depleted upper mantle, equilibrated at temperatures that were 200-250°C hotter (1000-1050°C) than the pyroxenites (80-900°C), which we interpret as cumulates related to basalt fractionation near the crust-mantle boundary. The higher temperatures recorded peridotite xenoliths imply a somewhat greater depth of burial and equilibration compared to the pyroxenites. These results indicate higher temperatures at moho depths beneath Kharchinsky Volcano, compared those inferred from studies of peridotite xenoliths at nearby Shiveluch Volcano (Bryant et al., 2007, G-Cubed). These differences may be related to different primitive melt genesis and evolution processes at these distinctively different volcanoes.